1. Field of the Invention
The present invention relates to an apparatus and method for reproducing signals from a magnetic optical disk.
2. Description of the Prior Art
Today, magnetic optical disks are coming into general use as a secondary mass storage for use with personal computers, workstations and the like. Signals are recorded on the magnetic optical disk as follows: In an appropriately biased magnetic field, a laser beam is applied to the magnetic optical film on the disk surface. The laser beam heats up the film where required to reverse polarity, forming pits that represent signals. The magnetic optical disk thus contains recordings at higher density than the conventional optical disk whose pits are physically formed depressions (or projections). In this respect, the magnetic optical disk is suitable for use as a mass storage.
FIG. 19 shows a typical relationship between pits recorded on the magnetic optical disk on the one hand, and a reproduced RF signal obtained by irradiating a laser beam to these pits on the other. As shown in FIG. 19, when the recording density is increased, one optical spot formed by the laser beam tends to irradiate two pits or more. As a result, higher recording densities leave the reproduced RF signal more likely to be affected by adjacent pits.
One solution to the above problem is to equalize the reproduced RF signal using a digital filter, as depicted in FIG. 20. In the example of FIG. 20, the input RF signal is delayed successively by "n" delay circuits 11 through 1n. The signals output by the delay circuits 11 through 1n enter "n" multipliers 21 through 2n which correspond to the respective delay circuits. After multiplication, the multipliers produce coefficients w1 through wn. The outputs of the multipliers 21 through 2n enter an adder 3 for addition. Coefficients w1 through wn from the multipliers 21 through 2n are then adjusted to a predetermined value. In this manner, the outputs of the RF signal from the adder 3 are equalized for a predetermined characteristic.
As signals are recorded over time on the magnetic optical disk, the pits recorded later tend to be larger than those recorded earlier due to such effects as heat response and accumulation, as illustrated in FIGS. 21 and 22. When the pits are read for reproduction, the larger pits proportionately raise the level of the RF signal. This means that the waveform of the reproduced RF signal no longer stays linear. In that case, equalizing the reproduced RF signal with the digital linear filter of FIG. 20 is not enough to obtain information accurately corresponding to the individual pits. Although it is conceivable to vary the coefficients "w" in accordance with the data (pit) pattern, this gives rise to a possibility of error propagation. Furthermore, attempts to vary the coefficients "s" in units of pits are unrealistic from a practical point of view.